Broadly speaking, polyimide films have been used as base films in flexible printed circuits (“FPCs”). These films are commonly called substrates. However, the marketplace often demands lower cost and higher FPC performance with each new generation of product, and therefore a long term need exists to lower base film cost while improving properties, particularly dimensional stability and flatness. In other examples of use, the polyimide films (or substrates) of the present invention are used as belts or as insulative tapes for wrapping electrically conductive wire.
Generally, “non-rigid” polyimide monomers are readily available and relatively inexpensive, but such monomers have traditionally formed polyimide substrates having poor dimensional stability. Such poor dimensional stability can cause a polyimide film to be problematic (if not impractical) for FPC applications.
U.S. Pat. No. 4,725,484, to Kumagawa et al., discloses polyimide compositions useful as FPC base films. However, the Kumagawa et al. patent teaches polyimides made from relatively expensive ‘rigid type’ monomers, such as, biphenyltetracarboxylic dianhydride (BPDA which is a relatively expensive monomer) and p-phenylene diamine (PPD).